The production process for a monolithic ceramic capacitor is generally as follows. First, sheet-form dielectric ceramic materials having a surface which is coated with an electrode material which becomes an internal electrode, are prepared. As the dielectric ceramic material, for example, a material made of BaTiO.sub.3 as the principal component is used. Then, the sheet-form dielectric ceramic materials coated with the electrode material are laminated by pressing under heat and by calcining the integrated laminate at a temperature of from 1,250 to 1,350.degree. C., a dielectric ceramic having internal electrodes is obtained. Also, by firing external electrodes to connect to the internal electrodes at the edge surfaces, a monolithic ceramic capacitor is obtained.
Accordingly, the material of the internal electrode is required to meet to following conditions:
(a) Because the dielectric ceramic and the internal electrodes are simultaneously calcined, the material of the internal electrode has a melting point which is the same as or higher than the temperature at which the dielectric ceramic is calcined.
(b) The material is not oxidized even in an oxidative high-temperature atmosphere and does not react with the dielectric ceramic.
As the electrodes meeting such conditions, a noble metal or alloy thereof, such as platinum, gold, palladium, silver-palladium alloy and the like, has been used. However, although these electrode materials have excellent characteristics, they are expensive, and are the largest factor increasing the production cost of monolithic ceramic capacitors.
Other high-melting materials include such base metals as Ni, Fe, Co, W, Mo and the like but these base metals are easily oxidized in a high-temperate oxidative atmosphere, whereby they become unusable as an electrode. Accordingly, to use these base metals as the internal electrodes of a monolithic ceramic capacitor, it is necessary to calcine the base metal together with a dielectric ceramic in a neutral or reducing atmosphere. However, conventional dielectric ceramic materials have the fault that when the materials are calcined in such a neutral or reducing atmosphere, they are greatly reduced and become semiconductor-like materials.
To overcome the fault described above, there are proposed, for example, a dielectric ceramic material wherein the barium site/titanium site ratio is in excess of the stoichiometric ratio in a barium titanate solid solution as shown in JP-B-57-42588 and a dielectric ceramic material made up of a barium titanate solid solution added with an oxide of a rare earth element such as La, Nd, Sm, Dy, Y, etc., as shown in JP-A-61-101459.
Also, as a dielectric ceramic material having a reduced temperature change of the dielectric constant, there are proposed, for example, a dielectric ceramic material of a BaTiO.sub.3 --CaZrO.sub.3 --MnO--MgO series composition as shown in JP-A-62-256422 and a dielectric ceramic material of a BaTiO.sub.3 --(Mg, Zn, Sr, Ca)O--B.sub.2 O.sub.3 --SiO.sub.2 series composition as shown in JP-B-61-14611.
By using such dielectric ceramic materials as described above, a dielectric ceramic which does not become a semiconductor-like material even when the material is calcined in a reducing atmosphere and the production of a monolithic ceramic capacitor using a base metal such as nickel and the like as the internal electrodes becomes possible.
With recent developments in electronics, the small-sizing of electronic parts has proceeded quickly and the desire to increase the capacity of monolithic ceramic capacitors has also become remarkable. Thus, the increase of the dielectric constant of a dielectric ceramic material and thinning of a dielectric ceramic layer have proceeded very quickly. Accordingly, the demand for a dielectric ceramic material having a high dielectric constant, showing a small temperature change of the dielectric constant, and having excellent reliability has become large.
The dielectric ceramic materials shown in JP-B-57-42588 and JP-A-61-101459 give a large dielectric constant but have the faults that the crystal grains of the dielectric ceramic obtained are large so that when the thickness of the dielectric ceramic layer in the monolithic ceramic capacitor becomes thin as 10 .mu.m or thinner, the number of the crystal grains in each layer is reduced, and the reliability is lowered. Furthermore, there is also a problem in the dielectric ceramic materials that the temperature change of the dielectric constant is large. Thus, the above-described dielectric ceramic materials cannot meet the requirements of the market.
Also, the dielectric constant in the dielectric ceramic material shown in JP-A-62-256422 is relatively high, the crystal grains of the dielectric ceramic obtained are small and the temperature change of the dielectric constant is small but because CaZrO.sub.3 and also CaTiO.sub.3 formed in the calcining process are liable to form a secondary phase with MnO, etc., there is a problem of reliability at high temperature.
Furthermore, there are faults in the dielectric ceramic material shown in JP-B-61-14611 in that the dielectric constant of the dielectric ceramic obtained is from 2,000 to 2,800 and that the material is disadvantageous from the view point of small-sizing and increasing the capacity of the monolithic ceramic capacitor. Also, there is a problem in that the dielectric ceramic material cannot satisfy the X7R characteristics prescribed by the EIA standard, that is, the characteristic that the changing ratio of the electrostatic capacity is within 15% in the temperature range of from -55.degree. C. to +125.degree. C.
Moreover, in the non-reducing dielectric ceramic disclosed in JP-A-63-103861, the insulation resistance and the temperature changing ratio of the capacity are largely influenced by the crystal size of BaTiO.sub.3, which is the principal component, whereby control for obtaining stable characteristics is difficult. Also, when the insulation resistance is shown as the product with the electrostatic capacity (i.e., CR), that product is from 1,000 to 2,000 M.OMEGA..multidot..mu.F and thus, it cannot be said that the non-reducing dielectric ceramic is commercially usable.
To solve the above-described problems, various components are proposed in JP-A-5-9066, JP-A-5-9067, and JP-A-5-9068. However, as a result of the requirement for further small-sizing and further increasing capacity, the requirements of the market for thinning the thickness of the dielectric ceramic layer and more severe reliability requirements, the need for a dielectric ceramic material having even better reliability and ability to cope with thinning the layer thickness have increased.
When a dielectric ceramic layer is simply thinned at a definite rated voltage, the field strength per layer is increased. Accordingly, the insulation resistance at room temperature and high temperature is lowered and the reliability is greatly lowered. Thus, it is necessary to lower the rated voltage when thinning the dielectric ceramic layer in conventional dielectric ceramic.
Accordingly, the necessary of providing a monolithic ceramic capacitor which does not require lowering the rated voltage even when the thickness of the dielectric ceramic layer is thinned, has a high insulation resistance under a high electric field strength, and has excellent reliability, has occurred.
To cope with surface mounting in a small-sized and large capacity monolithic ceramic capacitor, a plated film of a soft solder, etc., is formed on the external electrode formed by baking an electrically conductive metal powder. An electrolytic plating method is generally used as the method of forming the plated film. In the case of forming the plated film on the fired electrode, there is a problem in that when the monolithic ceramic capacitor is immersed in the plating liquid, the liquid enters through the voids of the baked electrode and reaches the interface between the internal electrode and the dielectric ceramic layer to lower the reliability.